A conventional sensor device such as a gyro sensor is equipped with a pair of right and left vibrators and an electronic sensor circuit. In the sensor circuit, detection signals of the vibrators are applied to a voltage conversion circuit through a charge amplifier and to a differential amplifier circuit to obtain a differential output of the detection signals. The differential output is, then, applied to a synchronous detector circuit, a low-pass filter (hereinafter, LPF) and a zero point/sensitivity adjusting circuit so as to be used as a sensor output.
In the above sensor circuit, the differential output of the differential amplifier circuit is so set as to assume a predetermined value depending upon the purposes of using the sensor device. Here, however, the offset voltage of the differential output becomes often abnormal. For example, the offset voltage becomes abnormal in case a bonding wire, which electrically connects the vibrators to the charge amplifier, is broken.
Therefore, there another sensor circuit is proposed to have an abnormal condition detector circuit for detecting an offset voltage that becomes abnormal based on the differential output. Specifically, this circuit is constructed as shown in FIG. 6, wherein an input signal VIN represents a differential output of the above differential amplifier circuit and is an AC signal.
As shown, the input signal is fed to an LPF 100 and a window comparator 101, where the input signal is compared with an upper-limit threshold value VRH and a lower-limit threshold value VRL. No diagnosis signal is output when the signal that has passed through the LPF lies in a range between the upper-limit threshold value VRH and the lower-limit threshold value VRL. A diagnosis signal is output when the signal is outside the above range.
In the above conventional sensor circuit, in which the differential output that becomes an AC signal is smoothed through the LPF. The time constant of the LPF is set large, and hence detection of an abnormal offset voltage is delayed. Thus, it is not possible to detect the offset voltage that becomes abnormal only temporarily and converges within a time constant. These problems will now be described in detail with reference to FIGS. 7 and 8.
FIGS. 7 and 8 illustrate in detail the circuit constructions of abnormal condition detector circuits provided for the conventional sensor circuit shown in FIG. 6. In FIG. 7, the LPF 100 is of the non-inverting type and includes an amplifier 100a, a resistor RLPF and a capacitor CLPF. In FIG. 8, the LPF 100 is of the inverting type and includes an amplifier 100a, resistors RI, RF, a capacitor CF and a reference voltage source VREF.
If the amplitude of the input signal VIN is denoted as Va, its DC component as Vdc, its frequency as fd and its angular frequency as ωd, then, the angular frequency ωd is expressed as follows.ωd=2·π·fd  (1)
Therefore, the input signal VIN can be expressed as follows.VIN(t)=Va·sin(ωd·t)+Vdc  (2)
The DC component Vdc is used for detecting abnormality in the offset voltage. In FIGS. 7 and 8, if the output voltage of the LPF 100 is denoted as VLPF and the voltage used as a reference voltage of LPF 100 in FIG. 8 is denoted as VREF (usually, VREF=VCC/2), the VLPF becomes as given below depending upon whether the LPF 100 is of the non-inverting type or of the inverting type.
In the case of the non-inverting type:VLPF=Vdc  (3)
In the case of the non-inverting type:VLPF=2·VREF−Vdc  (4)
These voltages are input to the window comparator 101 and are compared by two comparators 101a, 101b incorporated therein with the voltage VRH and with the voltage VRL. Here, the voltages VRH and VRL are provided by dividing a power source voltage VCC by voltage-dividing resistors 101d to 101f provided in the window comparator 101. The voltage VRL is lower than the voltage VRH. The comparator 101a produces a low level when VLPF becomes higher than the voltage VRH, while the comparator 101b produces a low level when VLPF becomes lower than the voltage VRL. Therefore, if either one of the comparator 101a or 101b produces the low level due to too high or too low VLPF, the AND circuit 101c produces a low level as a diagnosis signal indicating an abnormality.
The voltage VLPF relative to the input signal in FIGS. 7 and 8 is shown in FIG. 9. It is determined whether a difference ΔVdc between VLPF and VREF is in a range of voltages VRL to VRH (range of normal voltages) set with VREF.
In this abnormal condition detector circuit, it is desired that ripples (fd component) remaining after the smoothing dwindle. Further, the cut-off frequency fc of LPF 100 must be smaller than the frequency fd of input signals. It is, therefore, required to increase the time constant of LPF 100 resulting in a delay in detecting the abnormal offset voltage and, besides, making it difficult to detect the offset voltage that becomes abnormal only temporarily and converges within short periods of time of not longer than the time constant.